1. Field of the Invention
The present invention relates generally to gate valves and, more particularly, to apparatus and methods for a telescoping seat assembly with a uniquely operating seal arrangement.
2. Description of the Background
Telescoping seat assemblies have been known for use in gate valves for some time. One advantage of this type of seat assembly is the ability to provide a force on the seats and valve body that varies due to pressures encountered. In this way, one is assured of a good seal with a wide range of pressures. Another advantage is that friction due to movement of the gate may be kept at a minimum because the sealing force exerted by the seat assemblies may be automatically adjusted only as necessary to securely maintain a gas tight seal. Pressure lock conditions in telescoping seat assemblies have been solved in the past by varying the area of seal faces as discussed in at least one of the subsequent patents. However, it would be desirable to provide alternate or additional means for eliminating pressure lock. In addition or alternatively, it would be desirable to improve sealing over wider pressure, fluid type, and temperature ranges.
O-ring seals which use elastomeric material have the advantage of being easy to effect with loose tolerances. However, when a device may be used in a wide range of temperatures, fluids, and pressures, it becomes highly difficult to ascertain that the proper O-ring will be used. Numerous O-rings exist each with different fluid tolerances, pressure ranges, and temperature ranges. The charts which have all these effects are pages long and one must know the fluids, pressures, and temperatures for the application to select the correct O-ring. O-rings are also subject to the phenomena of explosive decompression when high pressures are suddenly reduced due to expansion of gas that has permeated into the O-ring. Thus, it becomes highly probable that in some cases depending on the fluid, pressure, and temperature the wrong O-rings will be used and the valve will leak.
Unidirectional seals made of non-elastomeric and non-permeable materials have been known that will operate under much wider ranges of temperatures, fluids, and pressures. However, such seals do not easily form a seal as do O-rings. The tolerances must be much tighter and the surfaces cannot tolerate imperfections. For this and related reasons, their use has been greatly limited. For the same reason, TEFLON based seals are not widely used even though they are inert to virtually any fluid. They cannot simply replace an O-ring.
An exemplary type of telescoping valve seat assembly gate valve is shown in U.S. Pat. No. 4,878,651, issued Nov. 7, 1989, to F. W. Meyer, Jr., which is incorporated herein by reference, as a through conduit gate valve apparatus with a valve seat assembly that is field maintainable and which prevents the occurrence of pressure lock. Each valve seat assembly includes a retainer ring fixed in the valve body and a pressure responsive seat ring that seals with the gate member. The seat ring is responsive to fluid pressure in the valve flow passageways for maintaining and enhancing the face seal with the gate and simultaneously to fluid pressure in the valve chamber for automatically venting the valve chamber to prevent occurrence of a pressure lock condition.
Another telescoping valve seat assembly gate valve is shown in U.S. Pat. No. 5,201,872, issued Apr. 13, 1993, to M. L. Dyer, which is also incorporated herein by reference, that discloses a gate valve wherein bore pressure in the body cavity of the valve is isolated in the open and closed position. In the closed position, a double metal sealing barrier is provided across the gate. In the open position, the pressure is isolated from the stem packing and bonnet gasket by the seats on either side of the gate. Formation of hydrates in the body cavity is eliminated, and better retention of body grease is achieved during flow through the valves. Metal-to-metal seal surfaces are developed, thereby eliminating wear on the non-metal components and reducing torque requirements.
U.S. Pat. No. 4,741,509, issued May 3, 1988, to Bunch et al., discloses a gate valve with a body having a valve member and gate chamber, passages communicating through the body into the chamber, a recess in the body surrounding the opening of each passage into the chamber, a bushing positioned in each recess, a gate positioned within the chamber, means for moving the gate within the chamber between positions communicating flow between the passages and closing flow between the passages, inner and outer unidirectional seals positioned between the surface of each recess facing the gate and the opposing surface of the bushing in the recess; each seal includes a U-shaped lip sealing element and a U-shaped spring positioned within the lip sealing element and urging the legs of the element apart into sealing engagement between the bushing and the body; the inner seal, which is positioned closest to the passage, has the open end of its sealing element facing the passage; and the outer seal, which is farther from the passage than the inner seal, has the open end of its sealing facing away from the passage; each seal and the body that has a depth less than the height of the seal while it is still in an effective sealing condition, the seals being compressible in height so that their bushings engage the body recess in a metal-to-metal seal when they are loaded and recoverable from such compressed position to provide a tight seal when said loading is relieved.
Thus, in some cases, it would be desirable to provide for the possibility of another method of venting excess pressure in the bonnet in a manner not shown by the prior art. As well, it would be desirable to provide for increased or wider range of pressure, fluid, and temperature operating conditions for a gate valves generally. In another application, it would be desirable if the downstream seat assembly were constructed to provide a backup seal if the upstream seat assembly were to fail. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.